Crohn's disease and ulcerative colitis are two major forms of human inflammatory bowel diseases (IBD). Their etiology is unknown, but increasing evidence indicate that immune mechanisms play an important role. It is well known that immune responses in the intestine remain in a state of controlled inflammation, suggesting that not only active suppression by regulatory T (TR) cells plays an important role in the normal intestinal homeostasis, but also its dysregulation leads to the development of IBD. This article reviews the unique aspects of TR cells and discuss how these control the intestinal homeostasis.